SANTA CLARA, Calif. (AP) Patrick Willis walked away first with a nagging toe injury that kept him from being the dominant All-Pro linebacker of his prime.

Then his heir apparent and San Francisco teammate Chris Borland followed with his own stunning retirement on the heels of his spectacular rookie season, citing concern about head trauma over a hard-hitting career.

Tennessee quarterback Jake Locker called it quits after four seasons. Next, ex-Pittsburgh pass-rushing specialist Jason Worilds bid farewell to football. And then yet another 49er joined the list of departures from the NFL while still young: Offensive lineman and 2010 first-round pick Anthony Davis also chose his health and future over more punishing knocks in the head after a concussion left him dazed for weeks late last year.

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''You don't want to see guys walk away, but at the end of the day everyone has their own problems and things they need to deal with, their own reasons,'' San Francisco tight end Vernon Davis said. ''We didn't expect Patrick to retire.''

Around the league, players began taking the leap to that unknown life after football - at 30 or younger, no less.

''As many players that do consider perhaps the long-term risks and the cost benefits of a long-term career in a contact sport, you're going to get that,'' said sports psychologist Dr. John F. Murray, based in Palm Beach, Florida. ''We've had more education and increased awareness from many avenues about the risks of concussions long term, the risks of the effects of that.''

In an offseason overshadowed by deflated footballs, Willis, Locker and the 27-year-old Worilds retired in a stunning 24-hour span starting March 10.

Five-time All-Pro Willis retired at age 30. Davis is 25 and Borland 24. Locker, then 26 - the NFL's eighth overall pick in 2011 - never played a full season and appeared in only 30 games in all.

Willis left without a Super Bowl ring, coming so close following the 2012 season in a three-point loss to the Ravens.

''I always told myself that I wanted it to be on my terms,'' Willis said in an emotional announcement at Levi's Stadium last spring. ''I wanted it to be in a way that was just amazing. ... In my head, I'm already a Hall of Famer. I am leaving this with closure, saying that I am happy today, more happy today than I was the day I was drafted. That says something to me.''

San Francisco players expressed mixed emotions at the turnover, as fearsome defensive end Justin Smith also retired, though the 35-year-old had 14 years in the league.

Borland and Anthony Davis feared concussions and head injuries.

''When I started there wasn't a whole lot of awareness on concussions,'' 40-year-old 49ers placekicker Phil Dawson said. ''Now, guys are informed. The doctors are on top of it. I think it's a good deal.''

Willis, San Francisco's defensive captain and locker room leader, explained his tender size-13 feet ''12 1/2 when they're bent'' could no longer handle the grind of NFL practices, let alone the demands of game day. He had surgery on his left big toe, went on season-ending injured reserve on Nov. 11 after getting hurt at St. Louis on Oct. 13.

''I have no regrets. I've had the most amazing eight years of football of my life,'' he said.

Locker has returned to his roots in Washington state with his wife and two young children.

Davis, the outspoken offensive lineman, left open returning if his body fully heals. Davis had been considering leaving for a few years, announcing his plans in a statement.

''This will be a time for me to allow my brain and body a chance to heal. I know many won't understand my decision, that's OK,'' Davis said. ''I hope you, too, have the courage to live your life how you planned it when day dreaming to yourself growing up. Your life is your dream and you have the power to control that dream. I'm simply doing what's best for my body as well as my mental health at this time in my life.''

For veterans who have stayed healthy, thoughts of retirement might be far from their minds.

''When you have those things going for you, why not keep playing?'' 38-year-old Raiders safety Charles Woodson said. ''Even though you've got guys retiring, there's a bunch of guys that would still love to be playing. For all of those guys that I've played with that tell me every year, `Keep going,' because they would love to have this opportunity.''

Murray says the NFL shouldn't be overly concerned about a dwindling talent pool.

''There will always be a demand for multi-million-dollar salaries and the glory that goes with playing NFL football,'' he said.

Still, constant change is part of the business.

''Every man in here has the right to decide how long he wants to play. It's his career,'' Dawson said. ''Whether it's retirements, or injuries or trades or cuts or whatever the case may be, for those of us who are still here you've just got to come to work and do the best you can.''

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AP Pro Football Writer Teresa M. Walker and AP Sports Writer Josh Dubow contributed to this report.

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